In a panoramic parallax stereogram, a plurality of two-dimensional (2D) views of a scene are recorded from a number of horizontally spaced-apart vantage points. The recording medium is commonly photographic film. The recording camera may be a single frame/single lens device with exposures made by translating the camera horizontally through a series of equally spaced vantage points from each of which the scene is photographed. Other techniques include photographing the scene using a multilens camera, a motion picture camera which is translated horizontally (during a short exposure burst of frames), a number of side-by-side cameras, and other commonly used methods. Having recorded the series of 2D images (normally on color negative film), it remains to combine these images into a final positive image for three-dimensional (3D) viewing. While several techniques are available for making 3D prints, the method used in the present invention employs lenticular technology, in which the 2D images are recorded in a photographic emulsion that is located behind a transparent lens sheet composed of vertically oriented, adjacent cylindrical lenses (lenticules). Each 2D image is "line-formed" --i.e., elements of all of the 2D images are contained within fine vertical lines behind each lenticule. In the final composing (exposure) of the composite image, the 2D frames are sequentially projected through the lenticular sheet over a series of horizontal angular zones. When viewing the composite image, the 2D frames will "feed back" to the viewer in the same angular zones in which they were originally exposed. Depending on the placement of the viewer's eyes, each eye will see only one of the original 2D frames, with each eye receiving a different frame and with the two frames viewed comprising a stereo pair. This entire process of recording the 2D frames and the final composing of the end product has been described in considerable detail in U.S. Pat. Nos. 3,895,867 (Lo, July, 1975) and 3,953,869 (Lo et al., April, 1976), to which reference may be made for a detailed description of techniques for composing 3D prints from 2D images on lenticular print material.
The 3D effect results solely from the difference in the horizontal displacement of points in the picture (parallax) with respect to objects in some reference plane in the two 2D frames (stereo pair) being seen by the viewer. The viewer's eyes and brain interpret and translate this parallax information into depth perception. Two frames and two eyes are required to complete this cycle.
A further understanding of parallax in the recorded frames can be obtained by reference to FIG. 1 where two spaced-apart lenses (L1 and L2) of equal focal length are shown imaging two points (A and B) as A' and B'. Three arbitrary spatial locations of A and B are shown in FIGS. 1-A 1-B, and 1-C. In FIG. 1A, A and B are placed an equal distance from the lenses with A on the optical axis of L1 and B on the optical axis of L2. x.sub.1 is clearly equal to x.sub.2 with B imaging on the same side of A from both lenses. In FIG. 1B, x.sub.2 is greater than x.sub.1, with B again recording on the other side of A. FIG. 1C demonstrates a location of A and B producing an x.sub.1 greater than x.sub.2 with B imaging on one side of A from L2 but on the other side of A from L1. This horizontal shift in image points as seen through lenses located in horizontally displaced vantage points will be collectively referred to as parallax.
In producing a panoramic parallax stereogram the number N of 2D frames employed can range from two to a larger number--e.g., 32. The optimum number of frames used is determined by such factors as the size of the final print, the anticipated viewing distance to the print, the spatial frequency of the cylindrical lenses in the print material, and the resolving power of the photographic emulsion. As a general statement, the larger the print and the longer the viewing distance, the lower the spatial frequency of the lenticules and the greater the number of 2D frames required. The greater number of 2D frames is required in order to reduce the angular width of the "feedback" zone of each frame which is needed at the greater viewing distances. Increasing the number of 2D frames in turn requires an increase in the width of each lenticule in order to stay within the recording capability of available photographic emulsions. For small prints up to say 5.times.7 inches in size (intended to be viewed at close, hand-held distances), four 2D frames are commonly recorded behind the lenticular surface, which may have a spatial frequency in the order of 200 lenses per inch; i.e., each lenticule is 0.005 inches wide. This provides a line width of 1.25 mils (32 microns) for recording a verticle element of each of the four frames as an image band in the photographic emulsion under each lenticule.
In recording the initial 2D images (of say an outdoor scene), each lens can be considered to record an infinite number of "object planes" in the scene from the nearest object and out to infinity (or to the furthest object in the scene). In combining the 2D images into the final lenticular stereogram, a particular plane is selected to appear (to the viewer) to lie in the plane of the print. Other planes will appear to be either in front of the print plane (foreground objects) or behind the print plane (background objects). In the object plane selected to appear to lie in the plane of the print, there is normally a prominent object, which will be referred to as the "key subject." FIG. 2 portrays (as an example) a 4-frame image set (N=4) to be used to compose a final stereogram. The asterisk (*) in each frame (1, 2, 3 and 4) is the image of the key subject (e.g., the nose of a person standing at mid-range in the scene). The stereo pairs (for subsequent viewing by the right and left eye of the viewer, respectively) are 1-2, 2-3, 3-4, 1-3, or 2-4.
Composing of the final stereogram is carried out by a printer. One task of the printer is to project the 2D frames across the assigned angular exposure zones to the lenticular recording material. While this may be accomplished through a series of "step and repeat" static exposures, the technique preferably used is either intermittently or continuously scanning, which is clearly explained in the patents referred to above and is also illustrated in FIG. 3. Again a set of four negative photographic images (#1, #2, #3, and #4) on a film strip is depicted as an example. Each negative image in sequence, beginning either with #4 and scanning left to right or beginning with #1 and scanning right to left, is separately projected by the enlarging lens onto the lenticular print film and scanned through an angle equal to the acceptance angle .alpha. of the lenticules (conventionally, .alpha.=30.degree.) divided by the number N of images. Thus image #4 is scanned through an angle from -15.degree. to 71/2.degree. by moving the film strip, the lens and the lamphouse proportionately from left to right. After all of the negative images have been scanned into the print film, the film emulsion bears the latent images of elements of all four negative frames as side-by-side image bands under the lenticules. The images under each lenticule are elements of each of the four frames occupying a width w equal to the total width W of the lenticule divided by the number of frames N (w=W/N).
The only correct way to understand the recording optics is to analyze the two discrete (and independent) imaging actions. In the first imaging action, shown at the left as "#1 (macro)" in FIG. 4, the enlarging lens images the 2D negative film frame on the surfaces of the lenticules. Consider it as an aerial image or better still as a modulated pattern of light with each .DELTA.x, .DELTA.y piece of lenticle receiving a given intensity and color illumination. It must be clearly understood that during the scanning exposure of each 2D frame, proportional motion of the film frame and the enlarging lens produces an absolutely stationary aerial image arriving at the lenticular surface at all angles during the scan. The only thing changing during the scan are the angles at which the aerial image arrives at each point on the lenticular surface. In the second imaging action, shown in the center as "#2 (micro)" in FIG. 4, the lenticule (cylindrical lens) unidirectionally images the exit pupil of the enlarging lens into the focal plane of the cylindrical lens )plane of the photographic emulsion). This second imaging action "line-forms" a vertical element of the frame into its assigned zone behind each lenticule. The "object" in this second (micro) imaging action is the exit pupil of the enlarging lens. As seen by any small .DELTA.x, .DELTA.y area of a lenticle it appears as a generally diffuse, uniformly illuminated disc whose color and intensity vary as a function of the content of the aerial image as seen from the .DELTA.x, .DELTA.y area under consideration. The cylindrical lens images in only one direction (at right angles to its long axis), which produces a generally elliptical exposure profile across the width e of the line image. This results directly from the area of the disc increasing from 0 at zone 140 to a maximum at zone 141 and back to 0 at zone 142 (elliptical function). A second result of the unidirectional imaging is that no detail is recorded across the line width e while any detail in the aerial image down the long axis of the lenticle is preserved. The line width e is determined as follows (refer to the right sketch in FIG. 4):
______________________________________ Let: .alpha. = acceptance angle of the lenticle .beta. = angle subtended by exit pupil from lenticle d.sub.EP = diameter of exit pupil s = long conjugate distance f = lens focal length F.sub.no = lens aperature (speed) m.sup.1 = enlargement ratio s = f(1 + m.sup.1) (1) ##STR1## (2) ##STR2## (3) ##STR3## (4) ##STR4## (5) ##STR5## (6) ______________________________________
For example, using an F/5.6 lens and an enlargement ratio of 10, .beta. is seen to be 0.93.degree.. Assuming lenticles with an acceptance angle .alpha. of 30.degree., .beta. is approximately 3% of the full acceptance angle--hence the need to scan (move the exit pupil over a range of angular positions) in order to expose 25% of W (assuming a 4-frame image set). The dimension e shown in the center view of FIG. 4 is 3% of 0.005" or 0.00015" (3.8 microns). The generally elliptical intensity within this fine line becomes essentially unimportant during the scan because of the continuously overlapping exposures it produces. The first function of the printer, then, is the sequential, proportional scanning of each 2D frame through a defined angular zone resulting in the line-forming of each frame.
A second and equally important function of the printer is the creation of the viewer illusion that the key subject plane appears to lie in the plane of the print material with foreground and background objects appearing to be in front of and behind the print plane, respectively. This function requires the performance of two actions by the printer and/or the printer operator: 1) The designation (typically by the operator) of the key subject which is to be presented to the viewer as lying in the plane of the print. 2) The registration by the printer of the key subject from all 2D frames at the print plane. This means that the key subject in the aerial image from each 2D frame must strike the plane of lenticular surface at the same x,y coordinate location. When the key subject from all 2D frames is registered in the plane of the print material, the viewer will then perceive this key subject to lie in the plane of the stereogram with foreground and background objects appearing in front of and behind the plane, respectively. This key subject registration is also shown in FIG. 3.
In one of the methods heretofore employed in printers to accomplish the second printer function just discussed, key subject registration, the first frame of the 2D set is picked up by a video camera and presented to the printer operator as a positive image on a video monitor. The operator then designates the key subject (to be printed in registration) using a screen cursor controlled by a joystick, roll ball, or other controller. Simultaneously, another frame of the set is analyzed for color content and density, from which the required lamphouse intensity and color and color balance are computed. After setting the lamphouse to the computed red, green, and blue light intensity and balance, the printing cycle is ready to begin. The first frame of the set is printed with no positioning correction, following which the printing is stopped to permit an operator-controlled registration cycle. A mirror is introduced into the printing path to deflect the aerial image to a secondary plane at 90.degree. to the print plane. A CCD video camera (without a lens) is moved by an x,y stage to the coordinate location of the designated key subject. The key subject image is captured by the photosensitive surface of the video camera, placed in digital memory (frame grabber), and presented to the operator as a positive video image on the monitor. The film is then advanced to the second frame of the 2D set, which is again intercepted by the mirror and the key subject video camera and presented on the monitor as a "live" negative image. The operator then uses a controller to move the film in the printer to bring the key subject in the second frame into registration with the "stored" location of the key subject in the first frame. (When coordinate registration is achieved, the negative video of the second frame cancels the positive video of the stored first frame and the screen is "greyed out.") Having achieved key subject alignment of frame 2 with respect to frame 1, the mirror is removed and the second frame is printed. This procedure is repeated for all remaining frames. The printing is seen to involve a serial operation of print, register, print, register, print, register, and print, the operator being required to perform the actual frame-to-frame registration. The printing time for each picture is typically 12 to 15 seconds, depending upon the skill of the operator. The prints-per-hour rate is in the order of 250. The printing is also very operator-intensive with operator fatigue being a significant negative factor.
An optional approach used heretofore uses a complete registration cycle for all 2-D frames to be printed (again with manual frame-to-frame registration by an operator with all registration data stored digitally) after which a printing cycle is performed. Total cycle time for each print remains high--in the range of 11 to 14 seconds--again depending upon the skill and fatigue level of the operator.